The present invention relates to printers and particularly to electromagnetically-operated print hammer assemblies for high speed impact printers.
Many different types of electromagnetically-operated impact printers have been proposed. Such impact printers utilize one or more electromagnets or solenoids to impart a driving force to a print hammer to drive the hammer against a print or type wheel, to return the hammer to its rest position after an impact printing, or both to drive the hammer to a print position and then to return it to its rest position after an impact printing. Examples of such proposed prior art impact printers, which comprise the background art known to applicant at the time of the filing of this application, are described below.
U.S. Pat. No. 3,335,659 discloses an impact printer in which a print magnet causes a print hammer to impact-print and then two electromagnetic coils are selectively energized to provide damping for the print hammer during its return movement.
U.S. Pat. No. 3,745,497 discloses an impact printer in which a pulse simultaneously energizes two electromagnetic coils to cause the front portion of an actuating arm to be pivoted upward and thrust an associated hammer pin upward to its printing position. At the end of the pulse, the coils de-energize, causing the front portion to be pulled down by a spring to its rest position.
U.S. Pat. No. 3,707,122 discloses an impact line printer in which each print hammer is normally maintained in a spring loaded position by an associated permanent magnet. When it is desired to print with a hammer, an associated coil is energized to produce an opposing magnetic field to counteract the field of the associated permanent magnet and allow associated leaf springs to propel the head of the hammer against an associated type wheel. Upon rebound of the print hammer, the associated coil is de-energized and the associated permanent magnet catches and locks the print hammer.
U.S. Pat. No. 3,705,370 discloses an impact printer which includes a three-legged core of magnetic material having upper, middle and lower legs. An armature, which is pivotally supported at its central portion adjacent to the middle leg, has a hammer face at its upper end spaced from the end of the upper leg to define an air gap therebetween and an operating winding around a pole piece mounted on its lower end abutting the lower leg. The upper leg of the core contains a permanent magnet or an upper winding to normally attract the upper end of the armature to its rest position against the permanent magnet. When the operating winding is pulsed, the holding effect of the permanent magnet or upper winding is overcome and the armature rotates to cause the hammer face to impact print paper and ribbon against a type wheel. When the pulse to the operating winding is terminated, the magnetic flux of the permanent magnet or upper winding restores the armature to its rest position.
U.S. Pat. No. 3,741,113 discloses an impact printer which includes first and second three-legged cores of magnetic material with a winding or coil on the middle leg of each core. An armature is pivotally mounted at one end thereof between the two cores. The armature has a hammer face at the other end and a projecting intermediate portion disposed to move within the winding on the first core when that winding is energized to enable the hammer face to impact a type wheel. The winding on the second core is later energized to damp oscillations and improve settle-out.
Because of the very tight quality requirements for MICR (magnetic ink character recognition) prints on bank checks and other financial documents, impact printing technology using total transfer type media, is the best method of printing MICR characters known to date. However, the long cycle time (settling time) of the impacting device (print hammer) imposes limitations on printing speed and thus the document throughput requirements.